The Structure of the Affinity Argument

Here's something that puzzles me about the Affinity Argument: What is its basic logical structure? From the way Socrates begins the argument, one would think that the argument should hinge on whether the soul is composite (syntheton) or non-composite (asyntheton). And yet the argument never returns to this notion, after it is introduced.

Socrates begins the argument (78b-c) by saying:

ou)kou=n toio/nde ti, h)= d' o(\j o( Swkra/thj, dei= h(ma=j a)nere/sqai e(autou/j, tw=| poi/w| tini\ a)/ra prosh/kei tou=to to\ pa/qoj pa/sxein, to\ diaskeda/nnusqai, kai\ u(pe\r tou= poi/ou tino\j dedie/nai mh\ pa/qh| au)to/, kai\ tw=| poi/w| tini\ ou)/>: kai\ meta\ tou=to au)= e)piske/yasqai po/teron [h(] yuxh/ e)stin, kai\ e)k tou/twn qarrei=n h)\ dedie/nai u(pe\r th=j h(mete/raj yuxh=j;

a)lhqh=, e)/fh, le/geij.

a)=r' ou)=n tw=| me\n sunteqe/nti te kai\ sunqe/tw| o)/nti fu/sei prosh/kei tou=to pa/sxein, diaireqh=nai tau/th| h(=|per sunete/qh: ei) de/ ti tugxa/nei o)\n a)su/nqeton, tou/tw| mo/nw| prosh/kei mh\ pa/sxein tau=ta, ei)/per tw| a)/llw|;

dokei= moi, e)/fh, ou(/twj e)/xein, o( Ke/bhj.

Well then, must we not ask ourselves some such question as this? What kind of thing naturally suffers dispersion, and for what kind of thing might we naturally fear it, and again what kind of thing is not liable to it? And after this must we not inquire to which class the soul belongs and base our hopes or fears for our souls upon the answers to these questions?

You are quite right.

Now is not that which is compounded and composite naturally liable to be decomposed, in the same way in which it was compounded? And if anything is uncompounded is not that, if anything, naturally unlikely to be decomposed?

I think that is true.

Given such a beginning, one would think the Affinity Argument should proceed by giving three different considerations (and there seem to be three considerations) for why the soul is not composite, thus:

1. Something perishes just in case it is composite.
2. It is likely that the soul is not composite, for three reasons:

(i) like other non-composite things, the soul is not perceptible through the bodily senses;
(ii) the soul has kinship with (i.e. naturally yearns to be with) non-composite things;
(iii) the soul naturally rules, as do only non-composite things.

3. Thus, it is likely that the soul does not perish.

And yet, as I said, that the soul is non-composite, or is like or akin to non-composite things, is not asserted anywhere in the argument, either in the course of the argument, or as a conclusion. The argument's conclusion is the following, which notably leaves out the very attribute on which the entire discussion was said to hinge:

…ta/de h(mi=n sumbai/nei, tw=| me\n qei/w| kai\ a)qana/tw| kai\ nohtw=| kai\ monoeidei= kai\ a)dialu/tw| kai\ a)ei\ w(sau/twj kata\ tau)ta\ e)/xonti e(autw=| o(moio/taton ei)=nai yuxh/,…

...this follows, that the soul is most like the divine and immortal and intellectual and uniform and indissoluble and ever unchanging.... (80b)

Sure, one can say that the notion of 'non-composite' is expressed in that of 'indissoluble' (or 'not liable to be dissolved'), however: (i) earlier Socrates had distinguished these two notions, and he proposed that indissolubility was a consequence of lack of composition, not identical to it; and, more importantly, (ii) it would still be the case that the logical structure of the argument is obscure, because, in the course of the argument, lack of composition changes from being that on which the argument hinges, to being one feature among many which a certain class of existences displays.